The use of chromium compounds in the polymerization of olefins is well-known. U.S. Pat. Nos. 2,825,721 and 2,951,816 teach the use of CrO.sub.3 supported on an inorganic material such as silica, alumina or combinations of silica and alumina and activated by heating at elevated temperatures to polymerize olefins. When these catalyst systems are used in various polymerization processes such as the well-known particle-form process, the resins produced, while useful in many applications, are unsatisfactory for others because of a deficiency in certain properties such as melt index.
Attempts to improve deficient properties of polyolefins produced using supported, heat-activated chromium oxide catalysts have been made by adding various metal and non-metal compounds to the supported chromium oxide prior to activation by heating. For example, in U.S. Pat. No. 3,622,522 it is suggested that an alkoxide of gallium or tin be added to supported chromium oxide prior to heat activation. U.S. Pat. No. 3,715,321 suggests adding a compound of a Group II-A or Group III-B metal to supported chromium oxide prior to heat treatment whereas U.S. Pat. No. 3,780,011 discloses adding alkyl esters of titanium, vanadium or boron and U.S. Pat. No. 3,484,428 discloses adding alkyl boron compounds.
In columns 5 and 6 and Table 1 of U.S. Pat. No. 3,622,522 the addition of aluminum isopropoxide to supported chromium oxide prior to heat activation is shown for purposes of comparison with the addition of an alkoxide of gallium or tin. The patentee concluded that the addition of the aluminum compound gave substantially the same or an increased HLMI/MI ratio of polymers produced as compares to the chromium oxide catalyst with no metal alkoxide added, whereas the addition of gallium or tin alkoxides produced polymers having a lower HLMI/MI ratio.
It is also known to utilize other chromium compounds as catalysts for the polymerization of olefins. Such compounds include various silyl chromate and polyalicyclic chromate esters as described, for example, in U.S. Pat. Nos. 3,324,095; 3,324,101; 3,642,749; and 3,704,287. The use of phosphorus-containing chromate esters in olefin polymerization catalysts has also been disclosed in the aforesaid U.S. Pat. No. 3,704,287; and in U.S. Pat. No. 3,474,080 and copending application Ser. No. 532,131, filed Dec. 16, 1974 now U.S. Pat. No. 3,985,676.
Use of the above chromium compound catalysts in Ziegler-type coordination catalyst systems has also been proposed. As is well-known in the art, such catalysts frequently additionally comprise organometallic reducing agents such as, for example, trialkyl aluminum compounds. Ziegler-type catalyst systems incorporating supported chromium compound catalysts and organometallic reducing agents, particularly organoaluminum compounds, are disclosed, for example, in U.S. Pat. Nos. 3,324,101; 3,642,749; 3,704,287; 3,806,500; and in the aforesaid copending application Ser. No. 532,131.